Radio receiving system



June 4, 1946.' J. L'. HYsKO RADIO RECEIVING SYSTEM Filed July 7, 1944.Hp /NVENTOR J. .HKSKO AAA VV Patented June 4, 1946 RADIO RECEIVINGSYSTEM John L. Hysko, Union, N. J., assigner to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication July 7, 1944, Serial No. 543,888

(Cl. Z50-20) 7 Claims. l

This invention relates to radio receiving systems and, moreparticularly, to an improved automatic frequency control circuit forcontrolling the frequency of waves generated by a beating oscillator ina radio receiving system.

Heretofore, automatic frequency control circuits have been employed inradio receiving systems for controlling the frequency of waves generatedby a beating oscillator having its output connected to a detector. IntheA case of radio transmission of telegraph signals employing waves ofone frequency for one type of signal, such as a marking signal, andwaves of a different frequency for another type of signal, such as aspacing signal, the receiving system usually employs band-pass filtersfor separating the two different frequencies. In some systems, thisfiltering is performed after the received radiofrequency Waves have beendetected and reduced to a lower frequency by combining them with wavesgenerated by a beating oscillator. If the frequencies of the receivedWaves depart from their assigned values, as they may due to variousreasons, then it becomes necessary to employ an automatic frequencycontrol circuit to Vary the frequency of the beating iscillator by acorrespending amount so that the frequency of the detected waves will beapproximately stable. However, in initiating communication with a radioreceiving station, the heterodyned Waves may, due to generation by thebeating oscillator of Waves of an incorrect frequency, be of frequencieswhich are outside the limits of the bandpass filters. When this happens,the output of the band-pass lters will be of nearly zero value and,consequently, will be insufficient to energize the automatic frequencycontrol circuit.

Accordingly, it is an object of this invention to provide improved meansfor synchronizing a radio receiving station with a radio transmittingstation.

It is also an object of this invention to provide a by-pass circuitacross the input to an automatic frequency control circuit for use whenthe frequencies of the heterodyned waves in the radio receiving systemare outside a preassigned range.

These and other objects of the invention are accomplished by connectinga monitoring electronic tube into a by-pass circuit shunting theband-pass input filter of an automatic frequency control circuit. Whenthe frequencies of the heterodyned Waves in the radio receiving systemare within the limits of the input filter, they are passed through thefilter and are amplified in the automatic frequency control circuit. Aportion of the amplied energy is utilized to bias the grid of themonitoring tube for rendering this tube non-conductive. However, if thefrequencies of the heterodyned waves are outsidethe limits of theband-pass filter, the biasing voltage will cease to exist therebypermitting the monitoring tube to become conductive. This, in eiect,completes a path in shunt to the filter and allows the monitoring tubeto relay the input waves to the amplifiers in the automatic frequencycontrol circuit. The resulting amplified energy then produces controlvoltages in the automatic frequency control circuit which are employedfor adjusting the frequency of the waves generated -by the beatingoscillator. These frequency adjusted waves are combined 4with the radioreceiver input waves and the resulting waves will have frequencieswithin the limits of the band-pass lter and consequently will passthrough the lter and will be amplified in the automatic frequencycontrol circuit. A portion of this amplified energy will be utilized, aswas mentioned above, to bias the grid ofthe monitoring tube to cut-othereby restoring the circuit to its normal condition. Thus, by means ofthis by-pass circuit and its monitoring tube, the admittance band-Widthof the automatic frequency control circuit is, in effect, substantiallyincreased.

These and other features of the invention are more fully described inconnection with the following detailed description of the drawing inwhich:

Fig. l is a block diagram of a radio receiving system employing theimproved automatic frequency control circuit; and

Fig. 2 is a circuit diagram of the improved automatic frequency controlcircuit.

In Fig. 1, radio signal waves, in the form of a radio-frequency carrierwave modulated by shifting its frequency either plus 425 cycles or minus425 cycles in accordance With marking and spacing telegraph signals, arereceivedvby the antenna l which delivers them to a radio receiver 2having a radio-frequency amplifier, oscillator, and converter 3 forconverting the received signal waves into waves having a frequency of450 kilccycles modulated to an extent of i425 cycles. Theselast-mentioned waves are amplified by an intermediate frequencyamplifier 4 having its output connected to an intermediate frequencyconverter 5 for converting these waves into waves having a frequency of50 kilocycles i425 cycles. The output of the converter 5 is supplied toa 50-kiloc-ycle modulator 6 in a receiver control unit 'I which alsocomprises a 400- kilocycle oscillator 8 and a 52,550-cycle oscillator 9,The 52,500-cycle oscillator 9 has its output connected to the50kilocycle modulator 6 for modulating the signal waves having afrequency of 50`kilocycles i425 cycles which are thereby converted intowaves of 2,550 cycles 1-425 cycles.

The output of the 50-kilocycle modulator 6 thus consists of waves havingfrequencies of either 2125 cycles, which is the marking frequency, or2975 cycles, which is the spacing frequency. This frequency separation,or difference, of 850 cycles between the marking and spacing signals maybe regarded as an invariant. These marking and spacing waves travel fromthe output of the 50-kilocycle modulator to a bandpass filter I having apass hand which is suiiiciently broad to pass both the Amarking andspacing waves but which eliminates noise currents having frequenciesoutside this range. The output of filter I0 is connected to a limiter IIin which the signal waves are amplified. The action of the limiter I Iis also such that it produces a substantially constant output for widevariations in the level of the input energy thereby compensating forvariations due to radio fading.

The output of the limiter II is connected to the marking filter I2 andthe spacing filter I3 in parallel. Filters I2 and I3 are designed topass the marking waves and the spacing waves, respectively, and toexclude waves of other frequencies. Filter I2 has its output connectedto the marking detector I4 and filter I3 has its output connected to thespacing detector I5. The detectors I4 and l5 amplify and rectify thesignal waves and then supply the rectified signals to the keyer controlcircuit I6 which causes the tone keyer I1 to produce an audio frequencysignal in response to the reception of marking energy and to produce nooutput in response to the reception of spacing energy.

'I'hese single-tone audio frequency signals produced by the tone keyerI1 are supplied to the send-tone equipment I8 from which they aretransmitted as ordinary single-tone voice frequency signals over anintermediate link I9 which may be either a land line or a radio circuit.The signals transmitted over the intermediate link I9 are received bythe receiving tone equipment 20 which, in turn, delivers them to thestation equipment 2I for such utilization as may be desired. Forexample, the station equipment ZI may be provided with receivingteletypewriter equipment adapted to record the signals.

As was stated above, the limits of the bandpass filter I0 are such as topermit the marking and spacing waves of 2125 cycles and 29'75 cycles,respectively, to pass. However, if the frequencies of the received radiofrequency waves have departed from their assigned values for any reason,such as due .to a shift in the frequency of the carrier wave, then theresulting audio frequency waves would be of frequencies which might beoutside the limits of the filter I0. In this event, some or all of theaudio frequency signal waves would be excluded by filter I0.

To avoid the occurrence of this undesired exclusion, the system employsan automatic frequency control circuit 23 having its input connected bya conductor 24 into the receiving system at a point between the outputof the 50kilo cycle modulator 6 and the input of the filter I0, as isshown in Fig. 1. The automatic frequency control circuit 23 produces adirect current control voltage in a manner explained hereinafter inconnection with the description of the operation of its circuit shown inFig. 2. This direct current control voltage is supplied over a conductor25 to the oscillator 8 for controlling, or varying, the frequency of thewaves generated by the oscillator 8 in a manner known to those skilledin the art. For example, the control voltage can be applied to a grid ofa reactance tube having a variable amplification factor so that, as thegrid becomes more positive, the gain of the tube is increased therebyincreasing the effective capacitance in the tuning circuit anddecreasing the frequency of oscillation. Negative potential on the gridwill, correspondingly, effect an increase in the frequency ofoscillation. When the frequency adjusted waves from oscillator 8 are nowcombined in the converter 5 with the waves from amplifier 4, thefrequency of the resulting waves will be of the proper value so that,when they are combined in the modulator 6 with the waves from theoscillator 9, they will produce marking and spacing waves of 2125 and2975 cycles respectively. Thus, the automatic frequency control circuit23 serves to maintain the frequencies of the signal waves, supplied tothe filter I0, at their proper preassigned values Within plus or minus50 cycles.

In Fig. 2, the marking and spacing waves fro the modulator 6 of Fig. 1are supplied to the automatic frequency control circuit-.over theconductor 24 as was described above. Since the 850- cycle frequencyseparation `between the marking and spacing waves may be regarded as aninvariant, as was also described above, the use of only one type ofsignal waves in the automatic frequency control circuit givessatisfactory operation. Since the marking wave is transmitted duringidle periods, it is preferable to use this type of signal wave tooperate lthe automatic frequency control circuit. Therefore, a band-passfilter 30 is employed to admit only the marking waves to the automaticfrequency control circuit. The marking waves will thus pass through thefilter 30, which may have limits of 1600 cycles and 2,600 cycles,`to thecontrol grid of tube 3| where they are amplified. These amplifiedmarking waves are then applied to the control grid of tube 32 where theyare further amplified. The output of tube 32 has two branches one ofwhich .delivers a portion of this output energy to the primary of atransformer 33. The secondary of transformer 33 delivers this portion ofthe output energy Vto a high-pass filter 34 vand a low-pass filter 35which ,are connected in parallel and which act as a discriminator.Filters `3M and 35 vare adjusted by means of the potentiometer 36 tohave the same losses for waves having a frequency of 2125 cycles.

The outputs of filters 34 and 35 are rectified by the tube 3l which is adouble diode connected to form two half -wave rectifiers. When themarking waves have a frequency of 2125 cycles, equal currents passthrough the two diodes of .tube 3'I and no output voltage will beproduced at the output terminals 38 and 39. If the frequency of themarking waves should be lower than 2125 cycles, this wave energy wouldpass freely through the low-pass filter 35 to the upper diode of tube 31but little or none of it would pass through the lower diode of tube 3l.When this condition exists, the upper output terminal 38 will have apositive potential with respect to the lower terminal 39. Since theupper terminal 38 is grounded, the lower terminal 39 will now benegative with -respect to ground. Similarly, if the frequency of themarking waves should be higher than 2125 cycles, this wave energy wouldpass through the high-pass filter 34 to the lower diode of tube 31.Under this condition, the lower terminal 39 will be positive withrespect to the upper terminal 38 and is, therefore, positive withrespect to ground. In either event, the output of tube 31 is applied tothe conductor 25.

As was stated above in connection with the description of the operationof the receiving system of Fig. 1, the control voltage from tube 31 isdelivered by the conductor 25 to some control means, such as a reactancetube, in the oscillator 8 for varying the tuning of the intermediatefrequency oscillator tube to maintain the output energy of the converter5 at substantially the proper preassigned frequencies.

As was also stated above, the output of tube 32 has two branches, one ofwhich has just been described above. In the other branch, a portion ofthe amplied waves travel along conductor 40 and pass through a smallcapacitance 4| of about 0.1 microfarad to a potentiometer, constitutedby the resistances 42 and 43, which impresses onehalf of the outputvoltage on the anodes of the double diode 44. The cathode of tube 44 isbiased about plus 23 volts by means of the battery 45 acting through apotentiometer 46. If the peak voltage of the alternating current outputof tube 32 is in excess of about 46 volts, for example, then pulses ofcurrent will pass to ground through tube 44 thereby increasing thecharge of the capacitor 4|. The resulting current in the resistors 43,41, and 48 causes the condenser 45 to acquire a negative charge.

One-half of the negative voltage on the condenser 49 is applied to thecontrol grid of tube 3|. Since tube 3l is a variable gain tube, its gainis thereby reduced. A condenser 50 is provided to reduce the voltageripple and also, together with the resistors 41, 48, and 5|, to delaythe response. Thus, tube 44 and its associated circuit act as anautomatic volume control means to maintain the output of tube 32substantially constant with a peak voltage a little greater than 46volts so that the signal input into the discriminator constituted by thelters 34 and 35 will also be constant. With this input to thediscriminator constant and invariable regardless of changes in thefrequency of the signal waves, then the output of the discriminator willbe unaffected by variations in the magnitude of the signal waves andwill be proportional only to variations in the frequency of the signalwaves.

Condenser 49 is also connected to the control grid of a monitoring tube52 through a delay circuit including the resistance 53 of about 1.5megohms and the condenser 54 of about 6 microfarads. When tube 32produces its normal output in response to the passage of signal wavesthrough the filter 30, the negative voltage on condenser 49 will besuflicient to bias tube 52 beyond cut-olf.

In initiating communication, a marking signal is sent from thetransmitting station and operates the automatic frequency controlcircuit as described above if the beating oscillator is initiallyoperating at the correct frequency. However, if the beating oscillatoris operating at an incorrect frequency, the heterodyned waves will be offrequencies outside the limits of the filter 30 and Will be shuntedaround the filter 30 through the tube 52.

These waves travel to the control grid of the monitoring tube 52 bymeans of a by-pass circuit including the conductor 55 and a resistance56 of about 4700 ohms. Because of the presence of the resistance 56, thelevel of the waves now applied to the control grid of tube 52 is onlyabout onesixth of the level applied to the input terminals of lter 30.Tube 52 will transmit this reduced wave energy over the remainder of theby-pass circuit, including the conductor 51 andthe resistance 58 whichis about 4700 ohms, to the control grid of tube 3i. Due to the loss inthe by-pass circuit, the level of the waves now supplied to tube 3| islower than the level of the waves ordinarily supplied thereto by filter30.

Tube 3l will .amplifyV this wave energy and will supply it to tube 32for further amplification. Due to the loss incurred in passing throughthe by-pass circuit, the output of tube 32 will be insuicient at thistime to elect a biasing of tube 52 beyond cut-off. In addition, any biasvoltage now applied to the control grid of tube 52 will reduce the gainof tube 52 and this in turn will reduce the value of the bias voltage.

The output of tube 32 is also applied to the discriminator constitutedby the filters 34 and 35. Sincey as was stated above, the output of thediscriminator is proportional only to variations in the frequency of theheterodyned waves, it will now produce a control voltage at its outputterminals 38 and 39. This control voltage will be applied by theconductor 25 to the controlled oscillator 8 in Fig. 1 for varying itsfrequency in the manner described above to bring the frequency of theoutput of the modulator 6 within the limits of lter 30. When thisoccurs, a sucient charge will again be placed on condenser 49 which willbias tube 52 beyond cut-oi, as was described above. This, in eiect,opens thebypass shunt circuit across the lter 30 thereby restoring thereceiving system to its normal condition.

'I'he operation of the automatic frequency control circuit during busyperiods of the radio communication system of Fig. 1 is as describedabove. However, if a protracted spacing signal is transmitted for aperiod of about 10 seconds, the delay circuit, comprising the resistance53 and the condenser 54, associated with the control grid of tube 52will discharge to permit tube 52 to function in the manner describedabove. As a result, the tuning of the controlled oscillator 8 will bechanged so that the frequency of the spacing Waves produced by themodulator 6 will now be approximately 2125 cycles instead of theirproper frequency of 2975 cycles. This condition can be corrected bytransmitting a steady marking signal for a period of about l0 seconds toallow the correct tuning to be reestablished.

It is to be understood that this improved automatic frequency controlcircuit can be used in other types of communication systems than thatshown in Fig. 1. For example, it can be used in a dual diversityreceiving system having two diversity receiving circuits each of whichis similar to the receiving circuit shown in Fig. 1. In this case, eachof these diversity receiving circuits can be provided with its ownautomatic frequency control circuit. However, if desired, only oneautomatic frequency control circuit need be used and this can beaccomplished by connecting its input into each of the diversityreceiving circuits and by connecting its output to a beating oscillatorwhich is common to both of the di-.

versity receiving circuits.

What is claimed is:

1. In a radio receiving station having a beat-` ing oscillator forgenerating waves and a converter for heterodyning received signal waveswith waves generated by the beating oscillator, automatic frequencycontrol means for regulating the frequency of the waves generated by thebeating oscillator, filtering means for supplying the automaticfrequency control means with a preassigned portion of the heterodynedreceived signal Waves, by-pass means for supplying other portions of theheterodyned received signal waves to the automatic frequency controlmeans, and control means for disabling the by-pass means in response tooutput energy produced by the filtering means.

2. A radio receiving system comprising in combination a beatingoscillator for generating waves, a converter for heterodyning receivedwaves with waves generated by the beating oscillator, automaticfrequency control means for regulatin-g the frequency of the wavesgenerated by the beating oscillator, filtering means arranged to supplythe automatic frequency control means with a portion of the heterodynedwaves, by-pass means arranged to supply at least another portion of thelieterodyned waves to the automatic frequency control means, firstcontrol means for disabling the by-pass means when the output of thefilter is above a preassigned level, and second control means forrendering said by-pass means effective when the output of said filterfalls below a preassigned level.

3. A radio receiving system comprising in combination a beatingoscillator for generating waves, a converter for heterodyning receivedwaves 'with waves generated by the beating oscillator, automaticfrequency control means for regulating the frequency of the wavesgenerated by the beating oscillator, filtering means ar' ranged tosupply the automatic frequency con,- trol means With a portion of theheterodyned Waves, by-pass means arranged to supply at least anotherportion of the heterodyned waves to the automatic frequency controlmeans, monitoring means adapted to disable the by-pass means when theoutput of the filter is above a preassigned level and for rendering theby-pass means effective when the output of said filter remains below apreassigned level for a preassigned period of time.

4. A radio receiving system comprising in combination receiving meansfor receiving radio frequency Waves, converting means for converting thereceived waves into corresponding audio frequency waves, said convertingmeans including a beating oscillator for generating Waves, automaticfrequency control means for varying the frequency of the waves generatedby the beating oscillator, said automatic frequency control meansincluding an input band-pass filter, at least one electronic amplifierconnected to the output of said filter, a by-pass circuit connectedbetween the input of the lter and the input of the electronic amplifier,and monitoring means adapted to close said by-pass circuit when theoutput of said electronic ampliiiel falls below a preassigned level andto open the by-pass circuit when the output of said electronic amplifierrises above said preassigned level.

5. A radio receiving system comprising in combination receiving meansfor receiving radio frequency waves, converting means for converting thereceived Waves into corresponding audio frequency waves, said convertingmeans including a beating oscillator for generating waves,

automatic frequency control means for varying the frequency of the wavesgenerated by the beating oscillator, said automatic frequency controlmeans including an input band-pass filter, at least one electronicamplifier connected to the output of said filter, a by-pass circuitconnected between the input of the filter and the input of theelectronic amplifier, a monitoring electronic tube connected in theby-pass circuit, a delay circuit having a condenser, charging means forapplying at least a portion of the output energy from the electronicamplifier to the condenser for charging same, biasing means forutilizing said output energy to bias the monitoring electronic tubebeyond cut-off for disabling the bypass circuit, and energizing meansfor rendering said monitoring electronic tube conductive for renderingsaid by-pass circuit effective when said output energy falls below apreassigned level and said condenser discharges through the delaycircuit.

6. A radio receiving system for receivingtwo types of signal waves ofdifferent frequencies, said system comprising in combination a Ibeatingoscillator for generating waves, modulating means for combining thereceived signal Waves with the waves generated by the beating oscillatorto produce signal waves of a first frequency and other signal waves of asecond frequency, automatic frequency control means adapted forproducing control voltages and for applying said control voltages to thebeating oscillator for varying the frequency of its output in accordancewith said control voltages, said automatic frequency control meanshaving an input band-pass filter connected to the output of themodulating means, said filter having limits adapted to pass signal wavesof only one of the frequencies produced by the modulating means, aby-pass shunt circuit adapted to be completed across the input band-passfilter, monitoring means adapted to complete the bypass shunt circuitacross the input filter, and activating means for activating themonitoring means to complete said by-pass shunt circuit during anabsence of substantial output energy from said lter.

7. A radio receiving system for receiving at least two types of signalwaves of different frequencies, said system comprising in combination abeating oscillator for generating waves, modulating means for combiningthe received signal waves with the waves generated by the beatingoscillator to produce first signal waves having a preassigned frequencyand second signal Waves of a different preassigned frequency, aband-pass filter having its input connected to the output of themodulating means and having limits for passing only said first signalwaves, control means connected to the output of said lter and adapted toproduce control voltages when the frequency of said first signal wavesdeparts from its preassigned value, means for applying said controlvoltages to the beating oscillator for varying the frequency of itsoutput, a normally open .by-pass circuit adapted to be connected aroundsaid filter, normally unoperated monitoring means adapted to close theby-pass circuit, and activating means for operating the monitoring meansduring the departure of said rst signal waves from their preassignedfrequency.

JOHN L. HYSKO.

